This project is concerned with the molecular mechanism by which ATP is hydrolyzed by the enzymatic action of muscle myosin and actomyosin. Since this mechanism is intimately related to structural changes which occur on the enzyme molecule upon binding ATP, our work is directed toward an understanding of these structural changes and correlation of these changes with the elementary kinetics steps of the hydrolysis reaction as catalyzed by myosin and actomyosin. Our approach to this general problem involves the use of stopped-flow and temperature jump relaxation techniques to investigate both rate-limiting and non-limiting reaction steps for the interaction of myosin subfragment 1 with its natural substrate and fluorescent analogs of the substrate, and to examine certain aspects of the interaction of myosin with actin under the influence of ATP. In parallel studies, we will investigate intramolecular distances of the myosin head and the effect of interaction with actin on these distances by using the method resonance energy transfer with specific fluorescent probes that are directed to specific sites witin the myosin head molecule. These studies are expected to contribute toward the elucidation of the myosin ATPase mechanism.